Electronic mail or “email” is an example method of exchanging digital communications between an author of the email and one or more recipients. Electronic communications are commonly transmitted over a network, such as a local network, the Internet and so forth. Electronic communication systems may enable users to perform actions related to the communication, such as creating, receiving, storing, forwarding and so forth. Email communications, for example, are commonly processed as follows. A user may compose a message using a mail user agent, including a recipient email address, which may be sent to an Internet service provider or other mail submission agent, which resolves the recipient email address to determine where to send the email. The email may be sent to one or more message transfer agents before reaching a message delivery agent. The recipient may then receive the email in an email mailbox. While this description represents one example email process, there are various alternative configurations and complexities which may also be utilized.
Many mail transfer agents have been exploited in the past to send unsolicited bulk email, commonly referred to as “spam”. Spamming, phishing, email bombardment, email spoofing and email worms are common issues that are faced by email providers and email users. Because the cost of sending email is small, spammers and other nefarious individuals may send hundreds of millions of email messages each day over an inexpensive Internet connection. This volume of email may be overwhelming for email servers and/or for email users. These emails may also contain malware programs causing a constant flow of unwanted email or causing more serious issues. Further, many malicious emails may pose a threat to an individual's privacy. Combating threats posed by spamming, malware and so forth to ensure a user's privacy and to reduce or eliminate unwanted email items in the user's email mailbox has been a focus of many efforts for a number of years.